bus error with GetBestVirtualSize()

Description

I have error a bus error on MacOSX when doing a
GetBestVirtualSize() on a wx.MenuBar generated by
xrced. See attached error dump. GetBestVirtualSize() on
other objects seem fine. Other platforms are ok.

The point of this bug is that calling wxMenuBar::GetSize() on OS X used to crash. If this is still the case, we must fix it. It doesn't matter that much what does the method -- if it could return the actual menu bar height, it would be better, of course, but if it returned wxDefaultSize it would still be better than crashing.